RTS Grid and usable bridges

Hi all, I have an rts project taking shape but I am trying to add bridges that can be crossed by all units the only problem is I am using a navmesh grid to determine passable areas but when I have an object placed for a bridge (currently just a cube) the area still stays red and units are unable to cross, any ideas on how to improve this and make bridges (cubes) passable would be greatly appreciated.

Thanks in advance for any help.

You can use Off-mesh Links to link together two separate navmeshes.